An optical network such as a passive optical network (PON) is a flexible access network that is capable of providing a range of broadband and narrow-band services for business and residential customers. The underlying equipment is considered to be relatively inexpensive for network operators because they do not require any active equipment or power supplies between the operator's central office (CO) and customer's premises (CP). As shown in the PON 10 of FIG. 1, downstream PON traffic is destined from the Optical Line Termination (OLT) 12 residing in the CO towards a number of Optical Network Terminals (ONTs) 16 (or Optical Network Units (ONUs), not shown), residing in the CPs via an optical splitter 14.
Current Gigabit-PON systems (termed GPONs) utilize scrambling techniques to reduce the probability of excessive number of consecutive ones or zeros in the digital transmission. This is necessary to keep the clock recovery mechanism of the receiver locked to the digital signal. The standard requires the receiver to tolerate up to 72 consecutive identical digits (CID). The upstream transmission of the GPON system operates in a burst mode, which requires fast locking at the start of the transmission and the ability to tolerate 96 CID for the remainder of the burst. The start of the burst contains a preamble with CID=1 to accommodate this. Burst mode receivers are more complicated, more costly and less available than continuous mode receivers.
One option is to utilize 8B10B or 9B10B line coding techniques. These techniques require additional bits (bandwidth) in the transmission. In the case of 9B10B coding there is a 10 percent penalty in bandwidth of the transmission medium. One of the proposals for the next generation 10 Gigabit Passive Optical Network (X-GPON1) uses 9B10B coding. The signal that is to be 9B10B encoded has forward error correction (FEC) encoding. The combined overhead of the 9B10B coding and FEC encoding is 25%. The proposal also requires a new transmission convergence (TC) layer standard.
What is desirable is a system and method for improved CID handling.